It has been very important in order to comply with environmental regulations in the United States to deal with the xe2x80x9cCluster Rulesxe2x80x9d established by the Environmental Protection Agency.
The Cluster Rules basically say that a cellulose pulp mill (e.g. kraft mill) has to collect streams of condensate resulting from the pulping operation containing at least 65% of the HAP""s (by EPA defined as MeOH which is a surrogate for real HAP""s which might be more than 50 different kinds of components) and to treat these condensates so that 92% of this HAP (MeOH) is removed and typically destroyed by conventional thermal oxidation, or the methanol is recovered therefrom in a conventional methanol recovery plant..
The mill will also elect to show that 7.2 lb/ODTP for unbleached pulp or 11.1 lb/ODTP for bleached pulp has been collected into a stream(s) which can be treated to remove 6.6 lb/ODTP and 10.2 lb/ODTP, respectively, with a maximum HAP (MeOH) concentration of 210 ppmv and 330 ppmv, respectively.
One good way to achieve Cluster Rules compliance is shown in copending application Ser. No. 09/248,006, filed Feb. 10, 1999, now U.S. Pat. No. 6,217,711. The invention provides an alternative and simple way to facilitate Cluster Rules compliance by enhancing the steam stripping operation.
According to the present invention, a method and system are provided for handling fluid streams, desirably foul condensate and treatment and product fluids associated therewith, that are advantageous over the art. The method and system of the invention achieve a significant energy saving, including reducing the amount of steam consumption, and achieve a significant cooling water saving, compared to conventional methods and systems.
According to one aspect of the invention, there is provided a method of handling fluid streams in a cellulose pulp mill using a stripping column and reboiler, comprising: a) Collecting foul condensate containing Hazardous Air Pollutants (HAPs) in a cellulose pulp mill. b) Feeding the foul condensate to the stripping column. c) In the stripping column, causing the foul condensate to flow countercurrently to stripping vapor so as to produce a HAPs-rich vapor and a clean condensate. d) Feeding at least part of the clean condensate from c) from the stripping column to the reboiler. e) Directing the HAPs-rich vapor from c) to enter the reboiler and at least partially condense while vaporizing clean condensate fed into the reboiler in d) to form a stripping vapor. f) Increasing the pressure of the stripping vapor from e). And g) using the increased pressure stripping vapor from f) as the stripping vapor in c). Usually c) is practiced to cause the stripping vapor to move upwardly while the foul condensate moves downwardly in the stripping column.
Preferably, f) is practiced to increase the pressure of the stripping vapor to about 0.1-30 psig (or any other narrower range within that broad range such as about 1-15 psig), most preferably about 5-10 psig. In a preferred embodiment, f) is practiced using a steam ejector. For example the steam ejector is supplied with clean steam at a pressure of between about 60-165 psig. The invention also preferably further comprises using the clean steam supplied to the steam ejector with the increased pressure stripping vapor from e) as the stripping vapor in g). Normally, the stripping vapor in c) is at a pressure about 0.1-30 psig.
The method may also further comprise h) feeding the condensed HAPs-rich vapor from e) to the stripping column. Preferably, h) is practiced by feeding the condensed HAPs-rich vapor to the stripping column above the area where the foul condensate in b) is introduced. In the method desirably part of the HAPs-rich vapor does not condense in the reboiler, and is fed from the reboiler to an external condenser to condense into condensate; and the condensate from the external condenser is combined with the condensed vapor from h) and fed to the stripping column.
Desirably clean condensate is formed in the reboiler; and the method further comprises using the clean condensate from the reboiler elsewhere in the pulp mill, such as in bleaching or washing of diluting of cellulose pulp, or slurrying comminuted cellulosic fibrous material prior to making pulp.
According to the invention, a)-g) are practiced so as to use less than 50% (e.g. about 20-25%) of the steam necessary to strip the same amount of foul condensate than if d)-g) were not practiced. Also, a)-g) are practiced so as to use less than about ⅓ (e.g. about 25-30%) of the cooling water to effect condensation in the external condenser than if d)-g) were not practiced.
According to another aspect of the present invention there is provided a method of handling fluid streams in a cellulose pulp mill comprising: a) Collecting foul condensate containing Hazardous Air Pollutants (HAPs) in a cellulose pulp mill. b) Causing the foul condensate to flow countercurrently to stripping vapor so as to produce a HAPs-rich vapor and a clean condensate. c) Vaporizing at least part of the clean condensate from b) using the HAPs-rich vapor from b) so that HAPs-rich vapor at least partially condenses and vaporizes clean condensate from b) to form a stripping vapor. d) Increasing the pressure of the stripping vapor from c). And e) using the increased pressure stripping vapor from d) as the stripping vapor in b).
According to yet another aspect of the present invention there is provided a system for handling fluid streams in a cellulose pulp mill comprising the following components: A stripping column having a bottom and top. A reboiler having a top and bottom. A conduit containing foul condensate containing HAPs collected from a cellulose pulp mill and connected to the stripping column at a first point between the top and bottom thereof. A conduit containing stripper overhead vapor extending from adjacent the stripping column top and connected to the reboiler at a second point between the top and bottom thereof. A first clean condensate conduit extending from adjacent the stripping column bottom, and a second clean condensate conduit extending from adjacent the reboiler bottom. A vapor pressure-increasing device. A vapor discharge conduit extending from a third point of the reboiler between the second point and the reboiler bottom, and connected to the vapor pressure-increasing device, And a conduit which transports pressure-increased vapor from the vapor pressure-increasing device to the stripping column at a fourth point between the first point and the stripper column bottom.
The system desirably further comprises an external condenser, and a conduit extending outwardly from the reboiler to the condenser. Also, preferably there is provided a first condensate-containing conduit connected from the external condenser to the stripper column at a fifth point between the fourth point and the stripper column top, and a second condensate-containing conduit connected from the reboiler to the fifth point.
While a wide variety of other conventional devices may be used, preferably in the system the vapor pressure-increasing device comprises a steam ejector. The system also preferably further comprises an indirect heat exchanger connected to the first and second clean condensate conduits and the conduit containing foul condensate including HAPs.
It is the primary object of the present invention to provide a method and system that comply with the Cluster Rules, and other environmental regulations, for handling HAPs in a cellulose pulp mill in an energy efficient and cost effective manner. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.